Track loop returns in addition to continue/break.

Besides `continue` and `break`, another way that a for loop can
exit early is via a `return` statement. This requires slightly
deeper inspection, because a `return` can be nested inside a
loop or switch.

Change-Id: I0a0e27d7f38dd51ffacb57a7a1f90657008d640b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/634457
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
7 files changed